The object of the invention, called a three-knife trimmer below, consists in cutting the printed products provided, in other words generally book blocks and/or brochures, at the three unbound sides. This takes place in that the book block or the brochure (only called a book block below) is clamped while stationary between pressing strips or pressing plates and the three above-mentioned sides of the book block are cut by means of three cutting devices. The cutting devices can be configured as counter-blade units, in which two blades cut against one another in the manner of scissors or as blade units having cutting strips, in which one blade cuts against a rigid plastics material strip, whereby for protection the knife edge easily penetrates into the plastics material strip in the end position.
In three-knife trimmers of this type, the head and tail are generally cut in a first phase and the front is cut in a second phase. However, the order is not imperative and it may also be carried out in reverse. It is furthermore possible to carry out only the head and tail cut or only the front cut on the book block, which, for example, is required for the production of English brochures.
There are three-knife trimmer configurations in which the book block remains stationary between the first cutting phase (for example head and tail cut) and the second cutting phase (for example front cut), and there are configurations in which the book block is transported between the cutting phases.
Three-knife trimmers have also become known in which the book blocks are pressed for cutting between pressing punches and cutting cassettes and are rigidly held for the cutting operation(s). When the pressing punch is raised, the cut book block is transported out and the next book block to be cut is introduced. The book block is brought into its position by a centering device and is then clamped by the pressing punch that is moving down. The blades move against the book with a swing cut and cut the unbound sides. After all the sides have been cut, the pressing punch is raised and the next work cycle can begin. This three-knife configuration is not, however, suitable for a rapid format changeover. The pressing punch and also the cutting cassette are customized for the format to be processed and can thus only be exchanged by stopping the machine.
A three-knife trimmer is disclosed in DE 102011105253 A1, in which the blades cut against cutting strips or counter-blades, wherein the book block is held by pressing strips next to the blades during a head and tail cut and during the front cut. A plurality of zigzag-shaped support webs for supporting the book block during cutting is arranged in the free space, which can be varied depending on the format, between the cutting strips or counter-blades, on the one hand, and the space between the pressing strips, on the other hand. Using a three-knife trimmer of this type, a good cut quality can be achieved because the book block is pressed or supported by the pressing strips and the zigzag-shaped support webs during cutting.
However, it has to be borne in mind with this solution that the book block can catch on the zigzag-shaped support webs during transport in and out of the cutting position. This circumstance is counteracted in DE 102011105253 A1, in that the transport system consists of a lower belt and an upper belt, the two belts for transporting the book block being moved together slightly with respect to one another so the book block cannot catch on the support webs of the cutting table plane or the pressing plane. However, with book block transportation of this type, the result is that with different book block thicknesses only one book block can ever be transported in the transport system. This restricts the permissible thickness difference from book block to book block because a plurality of book blocks is located in the transport system. In particular, with three-knife trimmers, in which the cut takes place at two stations, the transporting of book blocks that vary considerably with respect to thickness becomes a problem in the described transport system. With book block thicknesses which vary considerably, it is therefore necessary always to transport only one book block within the transport system. However, with a three-knife trimmer of this type, this restricts the capacity, i.e. the three-knife trimmer can only be operated at a low clock cycle (output).
Further three-knife trimmers, which define the prior art, are described in DE 102011105253 A1. However, none of them can satisfy the requirements for a quick changeover time linked with the requirement for a high cut quality.
A three-knife trimmer is disclosed in EP 1504860 A1, in which the book blocks to be cut are gripped by a positioning device and fed by an advancing device to the cutting devices. A plurality of cutting devices arranged at a distance from one another are provided, in which the book blocks are consecutively positioned in each case for a side cut by the advancing device. A side cut is carried out on the positioned printed product in each cutting device. The aligned book blocks are moved into a transfer position by the positioning device using an infeed gripper by means of a linear lifting movement, the alignment of the book blocks not being changed. The advancing of the book blocks takes place with a multiple planetary gear. Adjustable connecting control links are provided to position the book blocks in the cutting devices. The device allows an easy and rapid changeover to other formats. Each cutting device consists of a lower blade fixed to the frame and an upper blade, to which a pressing plate is coupled by means of a guide and a pneumatic cylinder. The pressing plate clamps the book block before cutting between the pressing plate and the stationary lower blade. In this case, the book block is not pressed over a large area, but only in the cutting region by the pressing plate and the lower blade, as well as the advancing device. The regions of the book block which are not pressed tend to “sag” and may therefore result in an unsatisfactory cut quality. This is particularly the case the book blocks consist of soft and/or thin paper.
A three-knife trimmer is described in JP2012-218114 A, which can process different book formats in succession and in which the outer sides of the book are not damaged. A clamping unit to grip the spine side of the printed product is attached to a moving part, the clamping unit having a reference face for positioning the spine of the printed product. Using a positioning actuator, the moving part is moved in a vertical plane by a controller and is positioned, in each case correctly for the format, for the cutting processes on the three unbound sides of the printed product, so that the cutting blades, which are moving in the horizontal direction, can cut the printed product. The printed product is aligned with the reference face of the clamping unit and a vertical contact face, as a result of which the controller is able along with the format data to approach the positions required for the respective cut and to correctly position the printed product for the cut.
The inventors have recognized that a drawback with this last mentioned three-knife trimmer is the limited possibility of varying the book format. Since the one clamping unit holds the printed product for all three cuts, the clamping unit has to be made substantially smaller than the smallest printed product to be processed. If the printed product has a substantially larger format, the plates, which are additionally used to support the printed product during cutting, have to be provided with a large hollowed-out region. A large hollowed-out region has a disadvantageous effect, however, on the cut quality.
For the front cut, the clamping unit can be inserted more or less deeply into the hollowed-out region of the plates. The printed product is only correctly supported when the clamping unit is guided deeply into the hollowed-out region of the plates. So that the printed product can be satisfactorily supported without changing the plates, only a small book width difference can be processed using the three-knife trimmer.